JPH0229197B

Core support base for a gas-cooled nuclear reactor of the pebble bed type comprises numerous bores for cooling gas (helium) and one or more discharge passages for the fuel spheres of uniform dia. The vertical bores for the cooling gas terminate at a distance below the upper face of the core support base; in the top face of the upper (graphite) blocks of this base, there is a large number of grooves running continuously across the base and communicating with the bores. For a pebble bed reactor where the core is composed of fuel elements of uniform size and reactivity control is by means of absorber balls of considerably smaller size (e.g. 1 cm.) poured into the core and trickling down among the fuel elements. The arrangement enables the primary circuit functioning to be maintained without any blockages of the gas passages and provides adequate security against mechanical damage when absorber balls are dropped into the core. As there are no circular holes of approximately the size of the balls, the bores cannot become blocked.